This patent application is directed at a current collector that works with an anode or a cathode of a lithium cell (lithium-ion cell, lithium-metal cell, lithium-ion supercapacitor) or any electrochemical cell that makes use of lithium as a charge storage vehicle. This application is not directed at the anode or the cathode itself.
When used in a lithium-ion cell or lithium-metal cell, this prelithiated current collector of the present invention provides additional (supplementary) or the primary source of lithium ions to be shuttled between an anode and a cathode, and extra lithium to compensate for the formation of solid-electrolyte interface (SEI) or other lithium-consuming mechanisms. The lithium-metal cell includes the conventional lithium-metal rechargeable cell, lithium-air cell (Li-Air), lithium-sulfur cell (Li—S), and the emerging lithium-graphene cell (Li-graphene, using graphene sheets as a cathode), lithium-carbon nanotube cell (Li—CNT, using CNTs as a cathode), and lithium-nano carbon cell (Li—C, using nano carbon fibers or other nano carbon materials as a cathode). Although not necessary, the anode and/or the cathode themselves can contain some lithium, or can be prelithiated prior to cell assembly.
When used in a supercapacitor (symmetric, asymmetric, hybrid, or lithium-ion capacitor), this prelithiated current collector of the present invention provides additional (supplementary) or the primary source of lithium ions needed to form electric double layers (EDL capacitance) and/or redox pairs (pseudo-capacitance) at the anode and the cathode, and/or the lithium ions to be shuttled between an anode and a cathode (e.g. to be inserted into a graphite or lithium titanate anode when a lithium-ion capacitor is recharged, and to be captured by surfaces of a carbon cathode when discharged). Although not necessary, the anode and/or the cathode themselves can contain some lithium, or can be prelithiated prior to cell assembly.
In the late 1980s and early 1990s, several safety incidents associated with earlier lithium-metal secondary batteries led to the abandonment of this class of high energy-density cells, paving the way for the emergence of lithium-ion secondary batteries. The pure lithium metal sheet or film (commonly used in the earlier lithium-metal cell) is replaced by carbonaceous materials as the negative electrode (anode) active material in the lithium-ion battery. The most commonly used carbonaceous anode material is graphite that is intercalated with lithium when the cell is charged. The resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1. In order to minimize the reduction in energy density due to this replacement, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge cycle of the battery must be minimized. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g. In real practice, the specific capacity of a graphite anode is typically much less than 355 mAh/g.
It may be noted at the outset that lithium is normally stored in the cathode (e.g. Li as part of LiCoO4) when a lithium-ion battery cell is assembled. This is due to the notion that cathode active materials are relatively more stable and prelithiated anodes are normally not air stable (very sensitive to oxygen and moisture in the open air). After the cell is fabricated, either the manufacturer or the user (typically the manufacturer) has to conduct the first charge cycle, bringing lithium ions out of the cathode active material (e.g. LiCoO4 particles), through the electrolyte, and into the anode active material (e.g. graphite particles). This important point will be further explained later.
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions. In particular, lithium alloys having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a<5) has been investigated as potential anode materials. This class of anode active materials has a higher theoretical capacity, e.g., Li4Si (3,829 mAh/g), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). Transition metal oxides, such as Co3O4 and Mn3O4, are also high-capacity anode active materials. An anode active material is normally used in a powder form, which is mixed with conductive additives and bonded by a binder resin. The binder also serves to bond the mixture to a current collector. Alternatively, an anode active material (e.g. Si) may be coated as a thin film onto a current collector.
The positive electrode (cathode) active material in a lithium-ion battery is typically selected from a broad array of lithium-containing or lithium-intercalated oxides, such as lithium manganese dioxide, lithium manganese composite oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel cobalt oxide, lithium vanadium oxide, and lithium iron phosphate. The cathode active material may also be selected from chalcogen compounds, such as titanium disulfate or molybdenum disulfate. These prior art materials do not offer a high lithium insertion capacity. The practically achievable specific capacity of a cathode material has been, for the most cases, significantly less than 200 mAh/g. Since the cathode specific capacity is relatively low, there is a strong desire to make use of a cathode active material to its full capacity.
As a lithium-ion cell is charged and discharged, lithium is alternately stored in the cathode and in the anode, so that the total amount of cyclable charges corresponds to the amount of lithium shuttling back and forth between the two electrodes. As indicated above, when the cell is assembled, usually the cathode active material is made to store the amount of lithium needed for the subsequent cyclic operation. This selection of cathode, instead of anode, to pre-store the needed lithium is mainly due to the notion that cathode active materials, such as lithium cobalt oxide, are relatively stable in ambient air (e.g., against oxidation) compared to lithiated graphite. However, the notion that this amount of lithium (that determines the battery capacity) is totally supplied from the cathode, limits the choice of cathode active materials because the active materials must contain removable lithium (thereby precluding the use of other non-lithiated materials that can be of significantly higher capacity). Also, delithiated products of LiCoO2 and LiNiO2 formed during charging (e.g. LixCoO2 and LixNiO2 where 0.4≦x≦1.0) and overcharging (i.e. LixCoO2 and LixNiO2 where x<0.4) are not stable. In particular, these delithiated products tend to react with the electrolyte and generate heat, which raises safety concerns. It is thus desirable to have a different way of supplying the needed lithium than using a cathode active material or a prelithiated graphite anode.
Further, when a lithium-ion cell is charged for the first time, lithium is extracted from the cathode and introduced into the anode. As a result, the anode potential is lowered significantly (toward the potential of metallic lithium), and the cathode potential is further increased (to become even more positive). These changes in potential may give rise to parasitic reactions on both electrodes, but more severely on the anode. For example, a decomposition product known as solid electrolyte interface (SEI) readily forms on the surfaces of carbon anodes, wherein the SEI layer comprises lithium and electrolyte components. These surface layers or covering layers are lithium-ion conductors which establish an ionic connection between the anode and the electrolyte and prevent the reactions from proceeding any further.
Formation of this SEI layer is therefore necessary. However, as the SEI layer is formed, a portion of the lithium introduced into the cells via the cathode is irreversibly bound and thus no longer participates in the cyclic operation. This means that, during the course of the first discharge, not as much lithium moves from the anode to the cathode as has previously been inserted into the anode during the first charging operation. This phenomenon is called irreversible capacity loss and is known to consume typically about 10% to 20% of the capacity of a lithium ion cell.
A further problem is that the formation of the SEI layer on the anode after the first charging operation may be incomplete and will continue to progress during the subsequent charging and discharge cycles. Even though this process becomes less pronounced with an increasing number of repeated charging and discharge cycles, it still causes continuous abstraction, from the system, of lithium which is no longer available for cyclic operation and thus for the capacity of the cell. Additionally, as indicated earlier, the formation of a solid-electrolyte interface layer consumes about 10% to 20% of the amount of lithium originally stored at the cathode, which is already low in capacity (typically <200 mAh/g). Clearly, it would be a significant advantage if the cells do not require the cathode to supply the required amount of lithium. It would be further advantageous if one could find an alternative way of safely and reliably supplying extra amounts of lithium to compensate for the initial and continued capacity loss.
One solution to this lithium supply issue being contemplated is to pre-store some (if not all) of the needed lithium in the anode. For instance, Takahashi, et al [“Secondary Battery,” U.S. Pat. No. 4,980,250, Dec. 25, 1990], Huang, et al [“Method for Fabricating Carbon Lithium-Ion Electrode for Rechargeable Lithium Cell,” U.S. Pat. No. 5,436,093, Jul. 25, 1995], and Jacobs, et al [“Rechargeable Lithium Battery Having Improved Reversible Capacity,” U.S. Pat. No. 5,721,067 (Feb. 24, 1998)] disclosed methods by means of which lithium is introduced into the anode active material in order to minimize the irreversible capacity loss. However, prelithiated anode active materials, such as carbon or graphite, lead to electrodes which can be handled only under non-oxidizing and dry conditions, making practical production of lithium ion batteries difficult. Yun, et al [US 2003/0039890] prepared an electrode (anode) layer of graphite particles bonded by a resin binder (PVDF) and then deposited lithium on the surface of graphite particles of this anode, as opposed to inserting lithium into the bulk of graphite particles. Nevertheless, lithium deposited on the exterior surface of an anode active material is just like a discrete piece of lithium metal, which is not air-stable and must be handled in an oxygen-free and moisture-free environment.
Meissner [“Secondary Lithium-ion Cell with an Auxiliary Electrode,” U.S. Pat. No. 6,335,115 (Jan. 1, 2002)] disclosed a secondary lithium-ion cell, which includes a carbon anode, a non-aqueous electrolyte, a cathode, and a lithium-containing auxiliary electrode disposed in the cell to compensate for the irreversible capacity loss. This auxiliary electrode is spatially separated from the electrolyte when the cell is positioned in a first orientation and contacts the electrolyte when the cell is oriented in a second position, for supplying additional lithium to the cell. Such an additional electrode makes the battery very complicated and difficult to make. Switching between two orientations is not a good strategy since it would complicate the handling of the battery and an average consumer would not pay attention to such a detail to ensure proper operation of such a battery.
The approach of using a separate, sacrificial electrode, in addition to an anode and a cathode in a cell, was also proposed earlier by Johnson, et al. [“Rechargeable Lithium Ion Cell,” U.S. Pat. No. 5,601,951, (Feb. 11, 1997)] and by Herr [“Lithium Ion Cell,” U.S. Pat. No. 6,025,093 (Feb. 15, 2000)]. Again, this additional electrode further complicates the manufacture and operation of a resulting battery. The assembling operation of a battery containing a highly reactive lithium metal or alloy electrode must be handled in an oxygen-free and moisture-free environment.
Gao, et al. [“Lithium Metal Dispersion in Secondary Battery Anode,” U.S. Pat. No. 6,706,447, Mar. 16, 2004 and U.S. Pat. No. 7,276,314 (Oct. 2, 2007)] disclosed a secondary battery containing an anode that is formed of a host material (e.g. graphite) capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material. The lithium metal is a finely divided lithium powder having a mean particle size of less than about 20 microns. The host material may be selected from carbonaceous materials (e.g., graphite), Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. The method of preparing such an anode includes the steps of providing a host material, dispersing lithium metal particles in the host material, and then forming the host material and the lithium metal particles dispersed therein into an anode. The lithium metal powder and the host material are mixed together with a non-aqueous liquid to produce a slurry and then applied to a current collector and dried to form the anode. The approach of Gao, et al has the following drawbacks:                (1) The anode is composed of an anode active material (e.g., graphite or Sn particles) and a discrete lithium metal phase (fine Li metal powder particles) forming a mixture of two types of particles. This implies that the anode still contains highly active lithium particles that are sensitive to oxygen and moisture and must be handled under very stringent conditions.        (2) Even when the lithium powder particles are surface protected (e.g. embraced by a layer of wax), this protective layer will react with electrolyte, significantly degrading the electrolyte performance or reducing the effective electrolyte amount. Further, there is a high tendency of breaking off this weak protective layer (exposing lithium to open air) during the electrode production procedure, thus significantly complicating the battery manufacturing operations.        (3) In a follow-on patent application, Gao, et al. [“Lithium metal dispersion in electrodes,” US Patent Application Pub. No. 2005/0130043 (Jun. 16, 2005)] suggested methods of lithiating an electrode prior to combining electrodes and other components to form a battery. In all cases, the electrode is composed of a mixture of discrete lithium metal particles or wire screen and powder particles of a host material, the latter being partially litiated. As shown in FIG. 1 of Gao, et al [2005/0130043], the anode comprises discrete lithium metal particles and a host material. Both the discrete lithium metal particles and lithiated carbonaceous material (graphite) are unstable in an oxygen- or moisture-containing environment. Furthermore, Gao, et al. have not fairly suggested how other anode active materials than graphite can be prelithiated in a controlled manner (e.g., without inducing a lithium coating on the surface of active material particles). In fact, no example was given to illustrate if or how other important anode active materials can be successfully prelithiated prior to battery production. No battery testing or electrochemical performance evaluation data was given in any of Gao's patents or patent application to demonstrate the advantages of their electrodes.        
Therefore, there exists an urgent need for a secondary lithium-ion battery that has one or more of the following features or advantages:    a) The battery does not contain a sacrificial electrode or an extra electrode in addition to an anode and a cathode in a cell;    b) The battery comprises an anode that does not contain an unstable lithium metal phase or lithium metal powder particles dispersed in the anode;    c) The battery contains no prelithiated anode active material that is not air stable and, hence, not conducive to battery fabrication in an ambient environment. Prelithiated carbonaceous anode materials (e.g., graphite, hard carbon, soft carbon, surface-modified graphite, chemically modified graphite, or meso-carbon micro-beads, MCMBs) are unstable in regular room air;    d) The battery comprises a convenient source of lithium (not disposed in an electrode active material) to compensate for the formation of SEI layers during charging, in addition to providing enough lithium to intercalate into a cathode active material during discharging.    e) The battery features a long and stable cycle life due to an adequate supply of lithium to compensate for potentially continued irreversible consumption of lithium.The present invention addresses all of the aforementioned issues associated with lithium-ion batteries.
Furthermore, as indicated earlier, today's most favorite energy storage devices (lithium-ion batteries) actually evolved from rechargeable lithium-metal batteries using lithium (Li) metal as the anode and a Li intercalation compound as the cathode. Lithium metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). This advantage of high capacity can be realized if the two major issues associated with the lithium metal anode can be resolved: (a) lithium metal instability in open air during cell fabrication and (b) dendrite formation during repeated discharges and charges. Dendrites are tree-like lithium structures that are formed during repeated charges and discharges, and often lead to internal shorting of a rechargeable lithium-metal cell (responsible for several lithium-metal cell failure-related incidents in late 1980s mentioned earlier). It is strongly desirable to avoid using a discrete layer of lithium metal (e.g. lithium foil) alone as an anode, or as a simple (unprotected) lithium coating on a copper current collector. The present invention also addresses this critically important safety issue.
In addition, a convenient and safe source of lithium is also essential to the operation of current and emerging high-energy or high-power lithium cells, such as the lithium-sulfur, lithium-air, lithium-graphene, lithium-carbon, lithium-carbon nanotube, symmetric supercapacitor, asymmetric ultracapacitor, hybrid supercapacitor-battery, or lithium-ion capacitor cell. The present invention also provides a surprisingly effective and safe solution to the lithium source problem in this wide variety of energy storage cells.
Thin lithium films are promising sources of lithium for the aforementioned applications. However, in addition to having the safety issue, very thin lithium foil is difficult and expensive to manufacture. Also, thin lithium foil, e.g., less than 20 microns thick, is too soft to have sufficient physical integrity for the production of cells and for strong connections with the required terminals. Lithium coated on a metal foil, such as copper, nickel, titanium, stainless steel, chrome plated steel and nickel plated steel, offers a good compromise between the desire for a very thin lithium layer and the requirement for sufficient physical integrity of the anode. But, the lithium coating layer remains sensitive to the oxygen, moisture, and nitrogen contents in open air.
Fauteux et al, [U.S. Pat. No. 4,935,317 (Jun. 19, 1990)] disclosed some typical composite cathode compositions and the utilization of lithium-coated metal foil as an anode. Many methods for coating lithium onto metal substrates are known in the art. For instance, Dremann et al. [U.S. Pat. No. 3,551,184 (Dec. 29, 1970)] proposed rubbing a heated metal substrate with a rod of lithium. Alaburda [U.S. Pat. No. 3,928,681 (Dec. 23, 1975)] disclosed Li coating of a metal substrate as it was conveyed through a lithium metal melt. Belanger et al. [U.S. Pat. No. 4,824,746 (Apr. 25, 1989)] disclosed a process of coating lithium or lithium alloy onto a metal substrate as the substrate is conveyed across a roller which is immersed in molten lithium or lithium alloy. A similar process is disclosed in U.S. Pat. No. 5,169,446 (Koksbang et al., Dec. 8, 1992). All these processes have to be conducted in a vacuum or protective atmosphere, but the coated substrate still has to be taken out of the coating equipment chamber and exposed to open air. No effective way of stabilizing the metal substrate-supported lithium coating film was disclosed in these studies.
Therefore, a need exists for an electrode structure that enables the electrochemical cell to operate with a high specific capacity, minimal irreversible capacity decay, and a long cycle life. In order to accomplish these goals, we have worked diligently and intensively on the development of new cell configuration, new electrode materials, and new current collectors. We have surprisingly found that a prelithiated current collector is a more versatile, more effective, and safer lithium source for all kinds of electrochemical cells, as opposed to using a prelithiated anode active material (not air stable), a free-standing lithium thin film (not air stable), or a simple lithium-coated metal substrate as an anode (not air stable).
In one preferred embodiment, this current collector is composed of a conductive substrate (e.g. Cu foil) coated with a layer of a mixture between carbon (as an example of a stabilizing agent) and lithium (or lithium alloy). By mixing lithium atoms with carbon (particularly amorphous carbon), we were able to form a mixture layer that is surprisingly air stable. Lithium atoms well dispersed in a disordered or amorphous carbon matrix, without forming lithium carbide, provides a stable source of lithium after a lithium cell is made. When the lithium content in the mixture layer exceeds approximately 80%, it is advantageous to deposit a thin layer of carbon to cover the mixture layer, completely eliminating the possibility of any air instability. Such a prelithiated current collector can be used in any cell that requires or needs a lithium source or an extra amount of lithium ions.
As compared to a prelithiated anode active material (e.g. prelithiated graphite), the prelithiated current collector is more advantageous in that, (a) the prelithiated current collector is more air stable, safer, and easier to handle; and (b) the prelithiated anode has a relatively limited lithium content (e.g. cannot exceed 355 mAh/g capacity) and, if portion of the stored lithium (out of this 355 mAh/g) is charged to enter a cathode active material, any parasitic or irreversible reaction occurring in the electrolyte or the cathode would serve to reduce the amount of lithium ions coming back to the anode during the subsequent recharge. Thus, the anode capacity would be lower than the initial capacity. If these irreversible reactions continue during subsequent charge/discharge cycles, the cell capacity will continue to drop. By contrast, the amount of lithium that can be pre-stored in the presently invented prelithiated current collector does not suffer from this limitation. The desired amount of lithium for the design capacity plus the anticipated need to compensate for the continued loss can be precisely pre-loaded into the current collector. This very subtle yet very significant advantage has never been recognized in any prior work.